1. Field of the Invention
The present invention relates to an air-conditioning device for electric automobiles for cooling and heating the interior of the passenger compartment of a car relying upon a refrigerating system and, particularly, for defrosting the heat exchanger on the outside of the passenger compartment.
2. Description of the Related Art
In general, the air-conditioning device of an electric automobile which is powered by a power source of secondary batteries effects cooling and heating based upon a refrigerating system which also serves as a heat pump. The refrigerating system that also serves as a heat pump comprises an internal heat exchanger installed in a blast duct that sends the air into the passenger compartment and an external heat exchanger for exchanging the heat with the external air that are provided in a coolant circulation circuit together with a compressor wherein, during the cooling operation, a high-temperature gas coolant supplied from the compressor is fed to the external heat exchanger so that the external heat exchanger works as a condenser, and the liquid coolant after having radiated the heat and is condensed is fed to the internal heat exchanger so that the internal heat exchanger works as an evaporator, in order to cool the air that flows through the blast duct utilizing the endothermic action of the internal heat exchanger. During the heating operation, on the other hand, the high-temperature gaseous coolant supplied from the compressor is fed to the internal heat exchanger so that the internal heat exchanger works as a condenser, and the air flowing through the blast duct is heated by the heat-radiating action.
During the heating operation, in this case, the external heat exchanger works as an evaporator. When the heating operation is continued for extended periods of time, therefore, the surfaces of the external heat exchanger become frosted and the heating ability gradually decreases. According to the prior art, therefore, the temperature or the like of the external heat exchanger is detected, presence or absence of the frost is judged based upon the detected value, and the defrosting operation is readily executed when it is judged that frost exists even when the vehicle is in operation. In the defrosting operation, the direction of coolant circulation is changed over to the opposite direction (same direction as during the cooling) to feed the high-temperature gaseous coolant supplied from the compressor to the external heat exchanger, and the frost is removed from the external heat exchanger by the radiation of the heat.
When it is judged that frost exists in the above-mentioned conventional constitution, the defrosting operation is readily assumed even when the vehicle is in operation. As shown in FIG. 7, therefore, the heating ability greatly drops during the defrosting operation; i.e., air of a greatly reduced temperature is blown into the passenger compartment due to the defrosting operation while the vehicle is in operation causing the passengers to feel uncomfortable. In addition, the external heat exchanger receives the cold air when the vehicle is running, whereby an extended period of time is required for the defrosting operation causing the passengers to feel more uncomfortable.
There has further been proposed another defrosting method which effects the defrosting operation while continuing the heating to a small degree by controlling the revolving speed of a coolant compressor in a sophisticated manner by using an inverter.
The passenger compartment of an automobile has a volume which is very small as compared with the ability of the air-conditioning device and, hence, the air blown from the air-conditioning device directly hits the passengers. Therefore, when the defrosting operation is effected by changing the mode to a weak heating, the lukewarm air that is supplied directly hits the passengers to give an uncomfortable feeling during the defrosting operation.
Moreover, the interior of the passenger compartment tends to be cooled very soon after the heating operation is discontinued since it is affected by the air in which the automobile runs and has a small heat-insulating efficiency. Therefore, when the defrosting operation is effected by stopping the heating operation, the interior of the passenger compartment is so cooled that the passengers feel uncomfortable.
The above problem can be solved by heating the air that is blown into the passenger compartment by using an electric heater during the defrosting operation or by developing a method which eliminates the need of effecting the defrosting operation while the passengers are on board.
When the electric heater is used during the defrosting operation, however, the electric power stored in the battery is consumed resulting in a decrease in the mileage which the vehicle is expected to run. Moreover, mounting the electric heater on the air-conditioning device results in an increase in the number of parts and cost for maintaining safety. It is therefore desired to develop technology that requires the defrosting operation as little as possible during the heating operation.
The present invention was accomplished in view of the above-mentioned circumstances, and its object is to provide an air-conditioning device for electric automobiles, which does not permit a drop in the temperature of the blown air that causes an uncomfortable feeling to the passengers despite the fact that the defrosting operation is carried out while the vehicle is in operation, in order to maintain comfortable heating. The present invention further provides an air-conditioning device for electric automobiles, which decreases the number of defrosting operations when the passengers are on board, and effects the defrosting operation without causing an uncomfortable feeling to the passengers as much as possible in the case where the defrosting operation must be effected while the passengers are on board.